In one common method for manufacturing plastic bags, a collapsed tube of thermoplastic film is provided in a longitudinal/machine direction. The collapsed tube proceeds downstream to a heat sealing and perforation station, which forms pairs of closely-spaced, parallel transverse heat seals at bag-length distances apart and, in addition, forms transverse perforation lines between each of the pairs of transverse heat seals. After leaving the heat sealing and perforation station, the collapsed tube is in the form of a plurality of handleless bags (pillowcases) interconnected along the perforation lines. The interconnected handleless bags are optionally folded and then fed to a cutting station for the purpose of forming a pair of handles in each of the plastic bags.
An example of one such cutting station includes a rotary anvil and a rotary cutting die mounted parallel and adjacent to each other. The rotary cutting die includes a cylindrical shaft and a blade mounted to the shaft. As each of the interconnected handleless bags pass between the rotary anvil and the rotary cutting die, the blade of the rotating cutting die contacts and cuts through the plastic material of each bag, thereby creating an incision in the bag.
One of the major components of cost in manufacturing plastic bags is the lower production speed associated with the aforementioned cutting station. The speed limitation in the cutting process pertains to the blade design of the rotary cutting die. A blade design that only cuts in the machine direction (i.e. direction of bag movement) has no speed limitations, while a blade design that cuts in both the longitudinal direction and transverse direction has a speed limitation directly related to practical blade life. The formation of plastic bags with handles requires a blade design that cuts in both the longitudinal direction and transverse direction.
Heretofore, the blade of the rotary cutting die has traditionally been shaped in the form of a U as shown in FIG. 8. The U-shaped blade creates an incision having a corresponding shape. When the incision is U-shaped, the legs of the U are along the handles of the bag and the base portion of the U is along the mouth of the bag. In the rotary die cutting process performed at the cutting station, the direction of the cut is from the bottom of the U to the top of the U, or vice versa.
During the cutting process, the blade of the rotary cutting die interferes with the adjacent rotary anvil. This interference can be measured as a force. During a single rotational cycle of the blade, the interference/contact force changes in relation to an area of the blade contacting the rotary anvil at a given time. The greater the area of the blade contacting the rotary anvil at a given time, the greater the interference force. The interference force must be sufficiently high throughout the entire cutting process to allow the blade to render a complete incision (i.e. cut fully through the plastic material of the bag). The interference force is at a maximum when the greatest area of the blade is contacting the rotary anvil.
A drawback of the prior art U-shaped blade design is that the difference between the maximum and minimum interference force during a single cutting cycle of the blade is quite large. This difference is depicted in FIG. 8, which schematically illustrates a U-shaped blade alongside a graph showing the variation in interference force between the blade and an adjacent rotary anvil. The machine direction is represented in FIG. 8 by an arrow labeled L, while the transverse direction is represented by an arrow labeled T. As a plastic bag travels in the machine direction L between the rotary anvil and the blade of the rotary cutting die, the interference force at a certain moment in time is directly proportional to the area of the blade contacting the rotary anvil. The area of the blade contacting the rotary anvil at a certain moment in time may be approximated by the area of intersection of a transverse line and the blade in FIG. 8.
While the blade is forming the base portion of the U-shaped incision, the area of the blade contacting the rotary anvil increases to a large peak and, therefore, the interference force increases to a peak of approximately 3600 pounds. This maximum interference force is required in order for the blade to render a complete incision. In contrast, while the blade is forming the legs of the U-shaped incision, the area of the blade contacting the rotary anvil at a certain moment in time is quite small and, therefore, the interference force is close to zero pounds. Like the U-shaped blade design, another prior art blade design schematically illustrated in FIG. 9 suffers from a large difference between the maximum interference force (approximately 1200 pounds) and the minimum interference force (close to zero pounds).
Unfortunately, in the prior art blade designs in FIGS. 8 and 9, parts of the blade of the rotary cutting die cannot effectively withstand the maximum interference force and are therefore worn down and eventually destroyed. This phenomenon sets specific limits on the practical processing speeds, particularly in the manufacture of handled plastic bags.
Another drawback of the U-shaped blade design in FIG. 8 is that it shortens the effective life of the rotary anvil. The legs of the U-shaped blade are perpendicular to the base portion, thereby causing a groove in the adjacent rotary anvil as the cutting process progresses.
A further drawback of the prior art blade design depicted in FIGS. 9 and 13 is that the plastic bag produced thereby is susceptible to tearing in the handle region. When the plastic bag is loaded with a product and then carried by its handles, the handles are stressed in a manner represented by the stress lines 80 in FIG. 13. These stress lines originate from the upper ends of the handles and diverge as they travel downward through the handles. The stress lines are such that the outermost ones 80a and 80b thereof are not confined to the handles, but rather leave the handles temporarily at points 82 and 84 before re-entering the handles near the lowermost ends thereof. This phenomenon of the outermost stress lines not being confined to the handles makes the handles somewhat susceptible to tearing along tear lines 86, which extend between the points 82 and 84.
A need therefore exists for an apparatus and method for forming handles in plastic bags which employs a blade design that reduces the difference between the maximum and minimum interference force between the blade and rotary anvil, that extends the effective life of the rotary anvil, and that renders the plastic bag produced thereby less susceptible to tearing in the handle region.